Light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent and fluorescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. However, the high cost of LED-based lighting systems has limited their widespread utilization, particularly in broad-area general lighting applications.
The high cost of LED-based lighting systems has several contributors. LEDs are typically encased in a package, and multiple packaged LEDs are used in each lighting system to achieve the desired light intensity. For general illumination, which utilizes white light, such white light may be generated in a number of ways. One approach is to utilize two or more LEDs operating at different wavelengths, where the different wavelengths combine to appear white to the human eye. For example, LEDs emitting in the red, green and blue wavelength ranges may be utilized together. Such an arrangement typically requires careful control of the operating currents of each LED, such that the resulting combination of wavelengths is stable over time and different operating conditions, for example temperature. The different LEDs may also be formed of different materials, for example, AlInGaP for red LEDs and AlInGaN for blue and green LEDs. These different materials may have different operating current requirements as well as different temperature dependencies of the light output power and wavelength. Furthermore, changes in light-output power with time may be different for each type of LED. Therefore, such systems typically utilize some form of active control of the current in each LED to maintain the light output power of each LED at the desired level. In some implementations one or more sensors (for example to sense light intensity, light color, temperature or the like) may be used to provide feedback to the current-control system, while in some other implementations the current may be adjusted over time based on values in a look-up table. Such control systems add cost and complexity to lighting solutions, as well as creating additional failure points. A further disadvantage of multi-LED arrangements is that they typically require some form of light combiner, diffuser or mixing chamber, so that the eye observes white light rather than the discrete different colors of each of the different LEDs. Such light-mixing systems typically add cost and bulk to lighting systems as well as reducing their efficiency.
White light may also be produced in LED-based arrangements for general illumination by means of light-conversion materials such as phosphors. LEDs generally emit in a relatively narrow wavelength range, for example on the order of about 20-100 nm. When broader spectra (for example “white” light) or colors different from that of the LED are desired, the LED may be combined with one or more light-conversion materials. An LED combined with one or more phosphors typically generates white light by combining the short-wavelength emission from the semiconductor LED with long-wavelength emission from the phosphor(s). This occurs because a portion of the LED light passes unconverted through the phosphor to combine with the phosphor-converted light. Phosphors are typically composed of phosphorescent particles such as Y3Al5O12:Ce3+ (cerium-activated yttrium-aluminum-garnet, or YAG:Ce) embedded in a transparent binder such as optical epoxy or silicone and applied as a layer.
In some implementations, the white light is a combination of the light emitted by the light emitter and the phosphor, while in other implementations the white light is substantially emitted by the phosphor alone (in this case the light from the light emitter is substantially not visible directly by the viewer). Herein, “white light” may be white or any other color that is produced by a combination of light from one or more light emitters and one or more light-conversion materials.
In some implementations, the phosphor layer absorbs a portion of the incident short-wavelength radiant flux and re-emits long-wavelength radiant flux. For an exemplary YAG:Ce phosphor, a blue LED may have a peak wavelength of 450 nm-460 nm, corresponding to the peak of the phosphor-excitation spectrum, while the phosphor emission has a broadband spectrum with a peak at approximately 560 nm. Combining the blue LED emission with the yellow phosphor emission yields visible white light with a specific chromaticity (color) that depends on the ratio of blue light to yellow light.
The geometry of the phosphor relative to the LED generally has a very strong impact on the uniformity of the resulting light. For example, the LED may emit from both the surface and the sides of the LED, producing non-uniform color if the phosphor composition is not uniform over these LED surfaces. To combat this problem, the LED may be placed in a reflecting cavity covered by a wavelength-converting ceramic, with the gap between the LED and the converter optionally filled with a transparent material, such that all of the light from the LED exits the cavity through the converter. However, ceramic wavelength converters may be difficult to manufacture and brittle in thin-film form. Furthermore, they may be expensive to integrate in arrays of small LEDs.
If the thickness of the phosphor layer, formed of a uniformly dispersed phosphor in a binder, is not uniform over the surface of the LED, relatively larger amounts of blue light will be present where the phosphor-infused binder layer is thinner and relatively smaller amounts of blue light will be present where the phosphor-infused binder is thicker. While techniques such as electrophoresis may be utilized to produce a uniform phosphor coating on an LED, electrophoresis typically requires a conductive substrate or an electrically conductive coating, thus increasing cost and complexity.
Phosphor integration typically takes two forms. In one approach the phosphor is integrated in the LED package along with the LED die, while in the other the phosphor is separate from the LED package or LED die. Packaged phosphor-converted LEDs, also known as packaged white LEDs, are typically fabricated by mounting the LED die on a portion of the package followed by integration of the phosphor. In some implementations, the phosphor is mixed with a binder or encapsulant formed over the mounted LED die and optionally over other portions of the package. It is desirable to be able to produce packaged white LEDs with relatively narrow distributions of electrical and optical characteristics, for example forward voltage, luminous flux, luminous efficacy, color temperature, color rendering index (CRI) and the like. However, there is a relatively large variation in these characteristics for the LED dies themselves due to the growth process for the epitaxial structure and the fabrication process of the LED dies. This variation is difficult to reduce, resulting in the need to test, sort and bin the LED dies for almost all applications. In other words, all of the LED dies are tested and grouped into bins related, typically, to forward voltage, light output power and wavelength. LED dies from specific bins are then chosen, based on the requirements of the particular application, put into a package, followed by formation of the phosphor around all or a portion of the LED die.
As explained above, in order to produce uniform optical properties from the packaged white LEDs, the phosphor process typically must be relatively uniform. In some implementations the phosphor, which is typically a powder, is mixed in a liquid binder and applied to the LED dies. It may be difficult to form a uniform layer of phosphor over the LED dies, resulting in different color temperatures for different packaged white LEDs. Furthermore, the phosphor powder typically has a significantly higher density than the binder, leading to settling of the phosphor powder during manufacture and a resulting variation in color temperature. In practice, packaged white LEDs are tested after manufacture and sorted and binned to produce groups of packaged white LEDs with uniform electrical and optical characteristics. It is clear that this is a complicated and costly process, with the potential for a significant portion of the output having non-optimal characteristics.
Conventional approaches may suffer from heating of the phosphor because the phosphor is in relatively good thermal contact with the LED die. As the phosphor heats up, it may lose efficiency and shift its optical properties, both of which are undesirable. One approach to at least partially mitigating heating of the phosphor is to provide increased thermal separation of the phosphor from the LED. Such an approach is sometimes called a “remote phosphor.” Remote phosphor configurations may be incorporated into packaged white LEDs by providing some physical and/or thermal separation of the phosphor from the LED die. One approach is to insert a portion of transparent binder or encapsulant between the phosphor-containing binder and the LED die. While such approaches at least partially mitigate phosphor heating, they result in a more complex and costly structure.
These issues may apply to many types of phosphor-converted light emitters, including single die-packaged devices, multiple die-packaged devices, arrays of packaged LEDs and single or arrays of unpackaged dies to which phosphor is applied.
In view of the foregoing, a need exists for structures, systems and procedures enabling the uniform and low cost integration of phosphors with LEDs.